Am I going to have trouble with my trap?

I am kind of nervous about one small part of my plumbing design, my code book shows this installation to be ok, but it just doesn't look right to me for some reason. Will my water heater drain trap be affected by the draining of the washing machine?

The problems I see there are the absence of a trap between the washing machine drain and the 4" line, and the fact that the trap for the water heater is going to be drying out all the time. So, I believe a trap between the washing machine drain and the 4" line would actually take care of both problems. And, I would come up to the washing machine with 2" pipe just to be sure the washer drain has plenty of vent and unrestricted flow.

There is no need for a trap between the WM and the 4", because you already have a trap on the WM. The water heater trap either should have, or must have, a trap primer to prevent drying out. One possible downside, is that if the line from the WM to the 4" becomes slightly restricted, or the WM has a high volume pump, there could be enough water to cause a backup through the floor drain during the discharge cycle. Unfortunately, the only way to check that is to install it and then try it, but by that time it is usually too late to do anything about it.

UPC Code in the Seattle area would require that the floor drain have it's own vent, rather than trying to wet vent a washer over a floor drain.

Wet venting requires upsizing the drain of the fixture being used as a vent, and that has not been done.

Like hj says, you will want some type of trap primer for the floor drain.
It may make sense to run 3" on the horizontal below ground for the washer, over to the 4" to prevent backing up into the floor drain.

The vent for the floor drain can tie back into the vent for the washer at 42" above the ground. In most cases, you can then go five feet to the trap.

There is no need for a trap between the WM and the 4", because you already have a trap on the WM.

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Maybe I am not up-to-speed here, but my own discharge from the washing machine is simply a 1-1/4" hose stuck down into a 2" pipe sticking up through the floor, and that pipe would function as a vent for the sewage line if it did not have its own trap beforehand.

908.2 The vertical piping between any two consecutive inlet levels shall be considered a wet-vented section. Each wet-vented section shall be a minimum of one pipe size larger.

A washer uses a 2" trap and waste. Upsizing would mean a 2.5" pipe. Or since it's hard to find 2.5" pipe and fittings, you would be looking at 3"
But since the code doesn't allow a washer on a wet vent, it's back to,
So how much would it cost to add a vent to the floor drain?
I think it's about $7.40

The reason for venting the floor drain, is to prevent the washer from siphoning the trap dry.

I am BC, Canada. And the code book was just updated a few months ago, so it's current for my region. The weather is fairly mild all year round here, so that's why I was trying to get away without using a primer, but I just looked in the code book regarding floor drains and it says I should be using a primer..so I guess that settles that (oh my poor wallet is imploding by now)

If what you are saying is true, then you have a completely improper drain system. Period.

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If so, everything is still exposed in the basement and I can easily change it, but what I have is all I have ever seen or known and it is essentially no different in design or function than if the washer's discharge hose would be hanging on the edge of a utility sink: the hose discharges into a line with a trap. So then, what is wrong there and/or what should I have instead?

The standpipe for your washing machine should be coming up from a trap. If it is not, you have a very bad situation as you will get sewer gas out of the standpipe. It is quite possible there is a trap below the floor, but that is not proper either. The trap for a washer is required to be ABOVE the floor. You need to have someone take a look at your situation,

Okay, take a look. The standpipe for my washing machine comes up from an eventual trap, and the location of that trap makes it impossible for sewer gas (or any other gas or odor or whatever) to ever come up out of the standpipe. In the picture, the trap you see is at the end of the run coming from the washer's standpipe (and nothing else), and that trap discharges into the 3" "wet vent"(?) that comes up out of my 4" line carrying gray water out of my house at the bottom of the picture. From what you have said, there is something wrong with my trap being "below the floor" and maybe even with it being so far away (about 15 feet), but there is no way the washer could ever blow the trap dry and there is no way even a heavy flow in the pipe could ever suck the trap dry.

Question: Why would it be required that the trap for a washer be "ABOVE the floor"? In my own case, that would make it succeptible to siphoning.